EAR-PF: The interaction between ductile shear zones and the seismogenic crust

Allison Kali L, Stanford CA

Investigators

Abstract

Dr. Kali L. Allison has been granted an NSF EAR Postdoctoral Fellowship to carry out research and education plans at the University of Maryland. The research project focuses on understanding the development of the structure of an intercontinental strike-slip fault, such as the San Andreas Fault in California and the Alpine Fault in New Zealand, and how it affects the earthquakes occurring on this type of faults. Faults like the San Andreas are highly localized in the shallow crust, and transition to broad zones of ductile deformation (shear zones) deeper in the crust. Earthquakes occur on the shallow, brittle portion, but they interact with the ductile shear zone. Examples of this interaction include the triggering of aftershocks by viscous flow, the spatio-temporal distribution of microseismicity, and microstructural data from exhumed faults. By simulating the development of the large-scale fault structure, and how it interacts with the earthquake cycle, this project will contribute to assessing the seismic hazard posed by active strike-slip faults. In particular, the depth of the transition from brittle to ductile deformation, which imposes a limit on the possible depth of earthquake rupture, and therefore is one control on the largest earthquake possible on a fault. The education plan will focus on the mentoring of an undergraduate student in research using sophisticated computational tools. Dr. Allison will also participate in public outreach and education activities at the University of Maryland. To investigate the interaction between the active faults and their ductile roots, the PI will develop a comprehensive numerical model that couples the shallow seismogenic zone and deeper ductile layers over the entire seismic cycle. The model will map microstructural processes likely to be significant in the lower crust, such as foliation and grain size reduction, to the scale of intercontinental faults. The project will also explore the effects of an inherited structure, such as a pre-existing anisotropic fabric in the lower crust or upper mantle, on the development of a ductile shear zone, and the spontaneous development of a fault through plastic deformation. It will also consider a broad range of time scales, ranging from fractions of a second during an earthquake rupture to millions of years during the development of an intercontinental fault. Thus, the project will connect our current understanding of the physical mechanisms considered, based on laboratory experiments and analytical work, to geodetic and geologic observations. Observables which will result from the model include: surface heat flux, the spatio-temporal pattern of surface deformation, and the average grain size in the shear zone. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.